A large number of chemical products, particularly chemical intermediates, are produced by selective, catalytic oxidation of an appropriate organic feedstock. One of the most important intermediates produced in this manner is ethylene oxide, which is made by oxidation of ethylene in the presence of a silver catalyst. The process operates in a loop, with modest conversion per pass, so that large amounts of unreacted ethylene are recirculated back to the reaction zone at each pass. The raw gas from the reactor is usually scrubbed with water to remove the ethylene oxide product before the gas is recirculated.
Similar processes are carried out in the manufacture of a number of other chemical products, where an oxidation reactor is used in a multiple-pass, loop mode, and where avoidance of build-up of an inert gas, particularly argon or nitrogen, in the reactor loop requires continuous or occasional purging. Examples of materials produced by selective oxidation include, but are not limited to:
Acetaldehyde, vinyl acetate and vinyl chloride, produced directly or indirectly from ethylene PA0 Propylene oxide and acrylonitrile, produced from propylene PA0 Benzoic acid, produced from toluene PA0 Caprolactam, produced from cyclohexane PA0 Maleic acid, maleic anhydride and phthalic anhydride, produced from various aromatic feedstocks PA0 Phenol, produced from cumene PA0 Terephthalic acid and dimethyl terephthalate, produced from p-xylene. PA0 (a) performing one or more reaction steps in a reaction zone to form an organic product, at least one of the reaction steps comprising a reaction of an organic feedstock and oxygen; PA0 (b) withdrawing from the reaction zone a crude organic product stream comprising the organic product, the organic feedstock and argon; PA0 (c) removing at least a portion of the crude product stream to form a non-product stream; PA0 (d) providing a membrane having a feed side and a permeate side, and being selectively permeable to the organic feedstock over argon; PA0 (e) passing at least a portion of the non-product stream across the feed side under conditions in which there is a pressure drop from the feed side to the permeate side; PA0 (f) withdrawing from the feed side an argon-rich purge stream enriched in argon and depleted in the organic feedstock compared with the non-product stream; PA0 (g) withdrawing from the permeate side an organic-feedstock-rich permeate stream enriched in the organic feedstock and depleted in argon compared with the non-product stream; PA0 (h) recirculating at least a portion of the organic-feedstock-rich permeate stream to the reaction zone.
The manufacture of acrylonitrile involves the reaction of propylene with ammonia and oxygen (usually from air), in the presence of a suitable catalyst, such as an oxide of bismuth, molybdenum, iron, nickel, or cobalt. The reactor off-gases are scrubbed, producing an aqueous acrylonitrile stream, which passes to further recovery and purification processes, typically steam-stripping and distillation. Scrubber off-gases are vented or recycled for recovery of the propylene.
Propylene oxide is manufactured by a two-stage process. The first is an epoxidation reaction of iso-butane and oxygen. The resulting tert-butyl hydroperoxide-butyl alcohol is reacted with propylene in the presence of a suitable catalyst, typically a molybdenum compound. The reaction mixture is separated and the crude product purified by distillation. The unreacted propylene is recycled for reuse.
Phthalic anhydride is produced by mixing o-xylene with compressed air and passing the mixture into a reactor containing a vanadium oxide or titanium-antimony oxide catalyst. The reactor off-gas is condensed, and the crude phthalic anhydride is purified under vacuum distillation.
Benzoic acid is produced by liquid phase oxidation of toluene in a stirred tank reactor, using cobalt naphthenate as catalyst. The reaction liquids are fractionated, and the crude product is rectified and further purified if necessary. Toluene is recycled from the fractionation step to the reactor.
Manufacture of phenol is a two-step process, first reacting cumene and sodium carbonate with oxygen in air to produce a cumene hydroperoxide. The crude hydroperoxide and a dilute acid are fed to a cleavage reactor, which produces phenol and an acetone by-product. The phenol is distilled and purified. Cumene from the first reactor is recycled back to the process.
Terephthalic acid (TPA) is produced in a similar manner to benzoic acid, by liquid phase oxidation of p-xylene using a cobalt salt catalyst. The reaction is carried out in an acetic acid solution, causing the TPA product to form a slurry at the base of the reactor. The reacted mixture is subjected to flash evaporation to remove the unreacted xylene and acetic acid, and the TPA slurry is centrifuged, crystallized and dried.
Dimethyl terephthalate (DMT) is made by the esterification of TPA with methyl alcohol to produce crude DMT, which is oxidized and distilled. Alternatively, DMT may be produced directly by a two-step process, first reacting p-xylene and recycled by-product p-methyl toluate in the presence of air and a catalyst. The intermediate product is mixed with methyl alcohol in an esterification tower. The resulting crude DMT is crystallized and distilled.
Caprolactam is also manufactured in a two-step process, first by a combustion reaction of sulfuric acid and ammonia. The resultant nitrosylsulfuric acid is reacted with hydrogen chloride and bubbled through cyclohexane in a photoreactor to form a crude caprolactam product. The crude product is further separated and treated to produce aqueous caprolactam. Cyclohexane is recovered from the separation step and recycled to the process.
Vinyl acetate is formed by the reaction of ethylene, acetic acid, and oxygen in the presence of a palladium salt or other suitable catalyst. The crude product is purified, and any unreacted ethylene is recycled to the reactor. The use of a membrane process for argon purging and organic recovery from vinyl acetate reactors is described in patent application (not yet assigned), incorporated herein by reference in its entirety.
Yet another related process is the oxychlorination of ethylene, in which ethylene, oxygen and hydrogen chloride are reacted in the presence of a fluid catalyst to produce ethylene dichloride. The reaction products are sent to a condensation step where the ethylene dichloride and water are removed. A portion of the remaining gas must then be purged and/or treated to remove excess argon, nitrogen, carbon dioxide and carbon monoxide before recirculation to the reactor.
Oxidation processes were originally developed using air as the oxygen source, but many modern processes operate with a feed of oxygen-enriched air or high-purity oxygen. The use of pure oxygen typically increases yields and reduces or eliminates the need for nitrogen purging from the process loop, since much less inert gas is fed into the loop initially.
Even when oxygen-oxidation is used, however, some purging is necessary. This is because "pure" oxygen is typically slightly less than 100% pure. The most significant other component is argon, with a typical concentration of about 1%. Argon is present in air and, since argon and oxygen have close boiling points, is not well separated in the cryogenic distillation process used to produce oxygen from air.
If argon is not removed, it builds up in the reactor loop, and can adversely affect the reaction dynamics and the flammability of the gas mixture, and/or reduce the life of the catalyst. Therefore, selective oxidation processes normally provide for a small purge stream to be withdrawn from the loop, usually after the crude product has been scrubbed out or otherwise separated. In addition to argon, the purge gas typically contains unreacted organic feedstock, oxygen, carbon dioxide, nitrogen, and small amounts of hydrocarbons and other contaminants. If methane has been added to the reaction zone to control the flammability of the gas mixture, methane may also be present in the purge gas. In prior art processes, this stream is incinerated or used as boiler fuel.
Although the volume of the purge stream is small, its destruction may result in the loss, from a typical plant, of many pounds of organic feedstock for every ton of chemical produced. In large-scale chemical intermediate processes such as those listed above, even incremental improvements in efficiency can affect process economics significantly. Therefore, a process that can reduce or eliminate this loss of organic feedstock would be valuable to the industry.
Patents that describe chemical intermediate manufacture by oxidation of organic feedstocks include: U.S. Pat. No. 5,179,215, to BOC, which discusses the manufacture of petrochemicals by selective oxidation and teaches the use of pressure or temperature swing adsorption to recover unreacted hydrocarbon feedstock. U.S. Pat. No. 5,278,319, also to BOC, further discusses the manufacture of petrochemicals by selective oxidation and teaches the use of a carbon dioxide removal system that also removes excess carbon monoxide. The hydrocarbon feedstock, now with the desired amount of carbon monoxide in the mix, may be recycled to the oxidation reactor.
A specific oxidation process, vinyl acetate manufacturing, is discussed in U.S. Pat. No. 3,444,189, to Union Oil Co., which describes the synthesis of vinyl acetate by oxidation of ethylene and acetic acid. Other references that teach the general manufacturing process for vinyl acetate include U.S. Pat. No. 3,557,191, to DuPont, which describes a process that uses ethylene to produce acetic acid, which is then reacted with additional ethylene to produce vinyl acetate. U.S. Pat. No. 3,547,983, to Air Reduction Co., Inc., describes the use of ethane instead of ethylene in the production of vinyl acetate.
Separation of certain gas mixtures by means of selective membranes has been known to be possible for many years, and membrane-based gas separation systems are emerging to challenge conventional separations technology in a number of areas. That membranes have the potential to separate organic vapors from other gases is also known. For example, U.S. Pat. Nos. 4,553,983; 4,857,078; 4,963,165; 4,906,256; 4,994,094; 5,032,148; 5,069,686; 5,127,926; 5,281,255 and 5,501,722 all describe membranes, systems or processes suitable for such separations.
U.S. Pat. No. 4,879,396, to Ozero, discloses a process for removing both carbon dioxide and argon from an ethylene oxide reactor loop by means of an argon-selective membrane, that is, a membrane that preferentially permeates argon and retains ethylene. U.S. Pat. No. 4,904,807, also to Ozero, discloses a process for removing argon from the reactor loop by means of an argon-selective membrane. In both cases, because the membrane is not perfectly selective, a portion of the ethylene is lost inevitably with the argon vent stream.